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    Viscoelasticity, nonlinear shear start-up, and relaxation of entangled star polymers

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    Type
    Article
    Authors
    Snijkers, Frank
    Ratkanthwar, Kedar cc
    Vlassopoulos, Dimitris
    Hadjichristidis, Nikos cc
    KAUST Department
    Chemical Science Program
    KAUST Catalysis Center (KCC)
    Physical Science and Engineering (PSE) Division
    Polymer Synthesis Laboratory
    Date
    2013-07-08
    Online Publication Date
    2013-07-08
    Print Publication Date
    2013-07-23
    Permanent link to this record
    http://hdl.handle.net/10754/562869
    
    Metadata
    Show full item record
    Abstract
    We report on a detailed rheological investigation of well-defined symmetric entangled polymer stars of low functionality with varying number of arms, molar mass of the arms, and solvent content. Emphasis is placed on the response of the stars in simple shear, during start-up, and for relaxation upon flow cessation. To reduce experimental artifacts associated with edge fracture (primarily) and wall slip, we employ a homemade cone-partitioned plate fixture which was successfully implemented in recent studies. Reliable data for these highly entangled stars could be obtained for Weissenberg numbers below 300. The appearance of a stress overshoot during start-up with a corresponding strain approaching a value of 2 suggests that in the investigated shear regime the stars orient but do not stretch. This is corroborated by the fact that the empirical Cox-Merx rule appears to be validated, within experimental error. On the other hand, the (shear) rate dependent steady shear viscosity data exhibit a slope smaller than the convective constraint release slope of -1 (for linear polymers) for the investigated range of rates. The broadness of the stress overshoot reflects the broad linear relaxation spectrum of the stars. The initial stress relaxation rate, reflecting the initial loss of entanglements due to the action of convective constraint release in steady shear flow, increases with Weissenberg number. More importantly, when compared against the relevant rates for comb polymers with relatively short arms, the latter are slower at larger Weissenberg numbers. At long times, the relaxation data are consistent with the linear viscoelastic data on these systems. © 2013 American Chemical Society.
    Citation
    Snijkers, F., Ratkanthwar, K., Vlassopoulos, D., & Hadjichristidis, N. (2013). Viscoelasticity, Nonlinear Shear Start-up, and Relaxation of Entangled Star Polymers. Macromolecules, 46(14), 5702–5713. doi:10.1021/ma400662b
    Sponsors
    We thank R Pasquino for help with the creep analysis and helpful discussions, as well as the reviewers of this work for their very contructive comments. Partial support from the EU (FP7 ITN DYNACOP Grant 214627) is gratefully acknowledged.
    Publisher
    American Chemical Society (ACS)
    Journal
    Macromolecules
    DOI
    10.1021/ma400662b
    ae974a485f413a2113503eed53cd6c53
    10.1021/ma400662b
    Scopus Count
    Collections
    Articles; Physical Science and Engineering (PSE) Division; Chemical Science Program; KAUST Catalysis Center (KCC)

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